I. Field of the Invention
The present invention relates generally to radial diffusers for a centrifugal compressor in a turbine engine, and more particularly, to a radial diffuser in which the vanes of the diffuser include a vortex fence to improve the efficiency of the diffuser.
II. Description of the Prior Art
The diffuser of a turbine engine distributes the pressurized gas flow from the turbine compressor to the combustor of the turbine. It is important that the diffuser obtains a high static pressure recovery and that the high velocity energy head produced by the compressor impeller is preserved from the pressure losses due to friction so that the total pressure of the gas is maximized as it reaches the combustor. As the total pressure recovery from these two pressure components increases, the efficiency of the turbine engine is increased.
One previously known device for obtaining a high static pressure recovery from the static energy and kinetic energy of the gases and for obtaining higher rates of diffusion is a vortex controlled diffuser. The vortex control concept has been studied when applied to the axial flow of gas from a first primary duct to a second coaxial and enlarged secondary duct. A radially extending vortex fence secured to the inner periphery of the secondary duct is axially spaced and radially spaced from the outlet end of the primary duct. A flow chamber upstream of the vortex fence forms a bleed mechanism that creates a vortex used to energize flow in the wake region immediately downstream of the diffusing step, whereby flow attachment on the diffusing roll of the secondary duct is enhanced. Low momentum fluid is removed from the upstream wall of the vortex fence and the corner vortex formed at the downstream end of the fence smooths flow transition to the downstream wall. The vortex control concept has been successfully applied to annular precombustor diffusers as has been reported by A. J. Verdouw in "Performance of the Vortex Controlled Diffuser (VCD) In an Annular Combustor Flow Path" in the AIAA Tech Info. Service Report A78-50574, March 1978, in R. C. Adkins and J. O. Yost's "A Combined Diffuser Arrangement" Technion-Israel Inst. of Tech. (Haifa), July 1979 and by John M. Smith in "Performance of a Vortex Controlled Diffuser in an Annular Swirl-Can Combustor at Inlet Mach Numbers up to 0.53", NASA Tech. Paper 1452, 1979. Nevertheless, the vortex controlled concept has not been previously found applicable to radial diffusers employed downstream of centrifugal compressors.
Previously known diffusers for centrifugal compressors have employed structural configurations of the vanes unrelated to the VCD concept in order to improve pressure recovery of the diffuser. For instance, the U.S. Pat. No. 3,788,765 to Rusak discloses wedge-type vanes having increasing thickness from inlet to exit wherein the pressure side of the vanes at the exit are streamlined to reduce the exit wake losses. The vanes include an extension at the trailing side of the exit vane to follow the shape of the streamlined pressure side of the following vane for better control of the channel exit area and the direction of flow. U.S. Pat. No. 3,658,437 to Soo discloses a diffuser wherein the opposing end wall surfaces converge and diverge within the vaned diffuser section. The cross section of the diffuser decreases and then increases in a manner suitable for maintaining fluid flow under a condition of imminent boundary layer separation. Although the U.S. Pat. No. 3,868,196 to Lown discloses a centrifugal compressor with a rotating vaneless diffuser in which the impeller establishes vortex flow in the diffuser, the tangential bleed through a clearance space between the impeller and rotating diffuser removes the high velocity leakage flow. Thus, the previously known radial diffusers do not apply the teachings of previously known vortex controlled diffusers where the low momentum fluid is removed at the upstream wall of the vortex fence and the vortex established smooths flow transition to the downstream wall.